Liquid laundry products such as liquid detergents and liquid bleach formulations have become increasingly popular in the last few years. However, aqueous liquid detergent formulations currently available do not contain a persalt bleach system such as is found in powdered detergents based on sodium perborate because of poor storage stability of the peroxide in the aqueous media. Although sodium perborate has been popular as a bleaching agent for powdered detergent formulations for many years in Europe, it has only recently found acceptance in the U.S. as a bleach for powdered detergents. There is a need, therefore, for stable, concentrated water-based persalt bleaching compositions which have a shelf life adequate to provide sufficient oxidizing peroxygen bleach in a commercial product. Such concentrated solutions are necessary so that a liquid laundry bleach, when diluted in the washing medium, will provide a concentration of active oxygen sufficient to provide adequate bleaching.
The solubility of sodium perborate in water at 20.degree. C. is 2.25% (corresponding to 0.23% active oxygen); however, it is known that the perborate solubility can be increased by use of solubilizing agents such as the alkali metal phosphates, boric acid, tartaric and citric acids as well as mineral acids, such as sulfuric acid and phosphoric acid. Although the perborate content can be increased by use of such cosolutes, the problem of adequate shelf stability remains of concern.
It has been proposed that the addition of a chelating agent or sequestrant can enhance the stability of sodium perborate in aqueous formulations by removing catalytic metal ions. Examples of such chelating agents or sequestrants include salts of ethylenediamine tetraacetic acid and complex organo-phosphates, such as the alkali metal salts of amino methylenephosphonic acid as disclosed in U.S. Pat. Nos. 3,234,140 and 4,477,390. The pentasodium salt of diethylenetriamine penta(methylene phosphonic) acid, which is available as DEQUEST.RTM. 2066 from Monsanto Chemical Co., is an example of such complex organo-phosphates.
Other stabilizer systems for peroxy solutions are inorganic salts of polybasic acids such as potassium polyphosphates, described in U.S. Pat. No. 3,553,140, quaternary ammonium salts described in U.S. Pat. No. 3,996,151, and picolinic or quinaldic acid which are described as stabilizers for organo peroxyacid bleach compositions in U.S. Pat. No. 3,956,159.
Recent patents and published applications disclose stabilization of hydrogen peroxide bleach compositions. See, for example U.S. Pat. No. 4,900,468 and PCT Application 0 209 228 which relate to use of substituted phenols as stabilizers. Also see PCT Application 0 349 153 which describes use of a stabilizing system comprising a heavy metal chelating agent and an aromatic amine free radical scavenging agent for hydrogen peroxide compositions. U.S. Pat. No. 4,900,469 discloses a hydrogen peroxide bleach solution stabilized with a chelating agent and an antioxidant. U.S. Pat. No. 4,238,192 describes a hydrogen peroxide bleach composition containing an alphaamino acid as a stabilizer. The compositions may also contain a small amount of dye or optical brightener. U.S. Pat. No. 3,970,575 claims a liquid hydrogen peroxide composition containing a phthalocyanine blue dye pigment. U.S. Pat. No. 4,430,236 discloses use of an anthroquinone blue dye in hydrogen peroxide. U.S. Pat. No. 2,012,462 discloses stabilization of peroxide solutions by use of a mixture of a salt of pyrophosphoric acid and an aromatic amine sulphonate in which the amino nitrogen may be substituted with an alkyl or aralkyl group.
Hydrogen peroxide solutions are generally quite acidic, having a pH in the range of about 1 to 4. This results in a more acidic washing solution, which can be a hazard to the user, as well as reducing detergency. The persalt solutions of this invention have a more neutral pH, which is desirable from the standpoint of safety and efficacy.